drm/sti: include linux/seq_file.h where needed

drm/sti: include linux/seq_file.h where needed

The sti drm driver has a lot of debugfs interface that cause
build errors in some configurations when seq_file.h is not
included implicitly:

drm/sti/sti_mixer.c: In function 'mixer_dbg_ctl':
drm/sti/sti_mixer.c:88:2: error: implicit declaration of function 'seq_puts' [-Werror=implicit-function-declaration]
drm/sti/sti_mixer.c:91:4: error: implicit declaration of function 'seq_printf' [-Werror=implicit-function-declaration]
drm/sti/sti_gdp.c: In function 'gdp_dbg_ctl':
drm/sti/sti_gdp.c:146:2: error: implicit declaration of function 'seq_puts' [-Werror=implicit-function-declaration]
drm/sti/sti_gdp.c:149:4: error: implicit declaration of function 'seq_printf' [-Werror=implicit-function-declaration]
drm/sti/sti_gdp.c: In function 'gdp_dbg_show':
drm/sti/sti_gdp.c:208:32: error: dereferencing pointer to incomplete type 'struct seq_file'

This adds an explicit #include statement in all of the affected files.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462830733-1710590-2-git-send-email-arnd@arndb.de